chat: attachment cache, download, and transfer hardening (Phase 3)

Phase 3 of docs/chat-hardening-plan.md — attachments can no longer turn
into unbounded memory, bandwidth, decoder, or task pressure (S15 closed;
S14's filename half closed).

Cache and image cost (3A): AttachmentCache now carries encoded- and
decoded-byte budgets (96 MiB / 64 MiB) on top of the count cap, with
per-entry weights, replacement accounting, and oldest-first eviction; an
individually over-budget fetch services any pending Save/Play from the
bytes in hand and is exposed as Evicted instead of retained.
validate_image_bytes prechecks header dimensions (per-side AND a new
14 MP total-pixel limit) before any decode; the renderer only ever
receives a ≤1600 px downscaled RGBA preview whose w*h*4 cost counts
against the decoded budget — originals stay encoded-only for Save.
sanitize_filename strips the bidi/zero-width spoofing set (RTL-override
extension spoof).

Download policy and state (3B): images auto-fetch only when roster-
authored AND declared ≤4 MiB, gated by a new deterministic
AutoFetchBudget (per-author and session request+byte token buckets,
check-then-take, bounded author map) alongside the existing dedup and
four-permit bound. Attachment state is now explicit — absence/Loading/
Ready/Failed/Evicted — driven by a new AttachmentFetchStarted event, so
skipped or evicted images render a "Load image" button instead of an
indefinite "loading…", and repeated clicks can never spawn duplicate
fetch tasks.

Exact transfers and serve store (3C): fetch_blob requires the received
length to equal the declared size (short = local error, overlong =
bounded-read reject, empty keeps meaning "sender no longer has it");
the file picker's unbounded read is replaced by a metadata-prechecked
cap+1 bounded reader; one Arc<Vec<u8>> now backs the UI cache, command
queue, and serve store; served_files is a count- and byte-budgeted FIFO
ServeStore (16 entries / 128 MiB).

37 new tests (568 lib total) including a real two-endpoint loopback
exercising exact/short/overlong/unknown-id transfers. Plan checkboxes
ticked and constant deviations decision-logged. Tests-green-only: the
plan's two-machine field-test section remains open.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/transport_loopback.rs

@@ -570,3 +570,94 @@ async fn dialer_connects_and_reconnects_without_an_address_lookup() {
         "audio should resume after reconnecting via the retained address; got {received} frames"
     );
 }
+
+/// A node that also serves the file plane (`FILES_ALPN`), mirroring how core
+/// registers the `FileRouter` for a session.
+async fn spawn_file_server() -> Node {
+    let lookup = MemoryLookup::new();
+    let endpoint = Endpoint::builder(presets::Minimal)
+        .secret_key(iroh::SecretKey::generate())
+        .relay_mode(RelayMode::Disabled)
+        .address_lookup(lookup.clone())
+        .bind()
+        .await
+        .expect("bind endpoint");
+    let transport = Arc::new(IrohTransport::new(endpoint.clone()));
+    let audio_router = AudioRouter::new();
+    audio_router.bind(&transport);
+    let file_router = peerspeak::network::iroh_impl::FileRouter::new();
+    file_router.bind(&transport);
+    let router = Router::builder(endpoint.clone())
+        .accept(AUDIO_ALPN, audio_router)
+        .accept(peerspeak::protocol::FILES_ALPN, file_router)
+        .spawn();
+    Node {
+        endpoint,
+        transport,
+        _router: router,
+        lookup,
+    }
+}
+
+/// Phase 3C: a file fetch must deliver EXACTLY the declared size — short,
+/// overlong, and unknown-id transfers are all rejected with local errors, and
+/// an exact transfer round-trips byte-identically.
+#[tokio::test]
+async fn file_plane_requires_exact_declared_size() {
+    let fetcher = spawn_node().await;
+    let server = spawn_file_server().await;
+    fetcher.lookup.add_endpoint_info(server.endpoint.addr());
+    server.lookup.add_endpoint_info(fetcher.endpoint.addr());
+
+    let server_id = server.endpoint.id();
+    // Member gating: the server only serves current room members.
+    server.transport.admit_audio_sender(fetcher.endpoint.id());
+
+    let blob = vec![42u8; 1000];
+    let id = [7u8; 32];
+    server
+        .transport
+        .serve_attachment(id, Arc::new(blob.clone()));
+
+    // Exact declared size: byte-identical round trip.
+    let got = fetcher
+        .transport
+        .fetch_blob(server_id, id, 1000)
+        .await
+        .expect("exact-size fetch succeeds");
+    assert_eq!(got, blob);
+
+    // Declared larger than served (short transfer): rejected, not cached as-is.
+    let err = fetcher
+        .transport
+        .fetch_blob(server_id, id, 2000)
+        .await
+        .expect_err("short transfer must fail");
+    assert!(
+        err.to_string().contains("incomplete transfer"),
+        "unexpected error: {err}"
+    );
+
+    // Declared smaller than served (overlong transfer): the bounded read
+    // rejects the stream rather than truncating it into a "valid" result.
+    let err = fetcher
+        .transport
+        .fetch_blob(server_id, id, 500)
+        .await
+        .expect_err("overlong transfer must fail");
+    assert!(
+        err.to_string().contains("read failed"),
+        "unexpected error: {err}"
+    );
+
+    // Unknown id: the empty body reads as the sender no longer having it.
+    let err = fetcher
+        .transport
+        .fetch_blob(server_id, [9u8; 32], 1000)
+        .await
+        .expect_err("unknown id must fail");
+    assert!(
+        err.to_string().contains("no longer has the file"),
+        "unexpected error: {err}"
+    );
+}